1. Field of the Invention
The present invention relates to a wiring board equipped with an insulating board which is made of a sintered product of silicon nitride having excellent heat-radiating property.
2. Description of the Prior Art
Accompanying the trend toward highly densely integrating the semiconductor elements in recent years, heat is generated in large amounts from a semiconductor device mounting the semiconductor elements, and it is becoming necessary to quickly radiate the heat out of the device so that the semiconductor device will not erroneously operate. For example, a power module equipped with a so-called power device such as MOSFET or IGBT has been used as a control board for electrically powered vehicles such as electric trains and electric cars. Such a control board deals with an electric current of several tens to several hundreds of amperes and a voltage of several hundreds of volts to supply a very large electric power. Therefore, the power device generates large amounts of heat arousing a serious problem in regard to how the heat that is generated be radiated out of the system to prevent the device from malfunctioning.
An insulating board made of a sintered product of alumina has heretofore been used for a package for containing the semiconductor elements or for a wiring board provided in a semiconductor device. However, the sintered product of alumina has a heat conductivity of as small as about 20 W/m.multidot.K and is not satisfactory from the standpoint of radiating the heat. Therefore, attention has been given to a sintered product of aluminum nitride having a high heat conductivity to substitute for the sintered product of alumina. However, the sintered product of aluminum nitride has such defects as a small strength and a small breaking toughness. That is, the semiconductor device equipped with an insulating board made of a sintered product of aluminum nitride cannot be used for a part that is subjected to a large stress and for which a high reliability is required even under severe conditions of use, such as the above-mentioned control board mounted on the electrically powered vehicles.
In recent years, the sintered product of silicon nitride is drawing attention as a material having a high heat conductivity and a large strength. Study has been conducted vigorously concerning the sintered product of silicon nitride as a high-temperature structural material (e.g., gas turbine) which must have a strength at high temperatures, and part of the study is arriving at a practicable region. That is, the sintered product of silicon nitride used as a high-temperature structural material exhibits excellent strength at high temperatures and a considerably high heat conductivity owing to its crystalline structure.
To obtain the sintered product of silicon nitride for use as the above-mentioned high-temperature structural material, however, it is necessary to use an oxide of a the firing at a temperature which is as very high as 1800 rare earth element as a sintering assistant and to execute to 2000.degree. C. to maintain a large strength at high temperatures. In order to suppress the decomposition reaction of the silicon nitride at high temperatures, furthermore, the firing must be conducted in a nitrogen atmosphere under a pressure of from several tens to a hundred atms. Accordingly, the study related to the radiation of heat of the sintered product of silicon nitride has been conducted in connection with, chiefly, those obtained through the firing at high temperatures under elevated pressure conditions. Heat-radiating properties of the sintered product of silicon nitride obtained through the firing at temperatures of not higher than 1800.degree. C. have not almost been reported. In other words, it has been known merely that a sintered product of silicon nitride can be obtained by using, as a sintering assistant, an oxide of a rare earth element and alumina through the firing at a temperature which is as relatively low as 1700 to 1800.degree. C., but its properties have not almost been known.
For example, Japanese Unexamined Patent Publications (Kokai) Nos. 135771/1994 and 149588/1995 teach sintered products of silicon nitride having heat conductivities of not smaller than 60 W/m.multidot.K and excellent heat-radiating properties by using at least an oxide of a rare earth element as a sintering assistant and through the firing in nitrogen under an elevated pressure at 1800 to 2000.degree. C. Japanese Unexamined Patent Publication (Kokai) No. 219731/1992 discloses a sintered product containing not less than 90% by weight of silicon nitride, containing aluminum and oxygen in amounts not larger than 3.5% by weight, respectively, having a density of not smaller than 3.15 g/cm.sup.3 and having a heat conductivity of not smaller than 40 W/m.multidot.K.
As described earlier, the sintered products of silicon nitride of the above-mentioned prior arts are obtained by a special firing method, i.e., firing in nitrogen under an elevated pressure condition at a temperature of not lower than 1800.degree. C., involving a defect of high firing cost. Despite the firing is conducted in nitrogen under an elevated pressure condition, it is difficult to completely suppress the decomposition of the silicon nitride. Therefore, the fired surface (as fired surface) of the product is very coarsened, requiring a cumbersome machining work such as polishing after the firing, resulting in a further increase in the cost of production.
On the other hand, the sintered product of silicon nitride obtained through the firing at a low temperature which is not higher than 1800.degree. C. is free from the problem in that the silicon nitride is decomposed during the firing. Therefore, this sintered product of silicon nitride is obtained through the firing in a non-oxidizing atmosphere under normal pressure (atmospheric pressure), offering such advantages that the cost of firing is cheap and the fired surface (as fired surface) is coarsened little. However, the sintered product of silicon nitride obtained through the firing at such a low temperature has a heat conductivity of from 20 to 30 W/m.multidot.K and radiates the heat in amounts considerably smaller than that obtained through the firing at a high temperature, which is a fatal defect. That is, when the sintered product of silicon nitride is produced through the firing at a low temperature, there are used an oxide of a rare earth element and alumina as sintering assistants, whereby aluminum atoms dissolve as a solid solution in the crystalline particles of silicon nitride to form sialon. As a result, the silicon nitride crystals exhibit a decreased heat conductivity.
On account of these reasons, the wiring board equipped with the insulating board of a sintered product of silicon nitride prepared through the firing at a low temperature, is far from being put into a practical use.